The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio at a minimum transmission ratio.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). The transmission ratio is automatically and continuously reduced at a speed which is decided by pressure of oil supplied to the servo device of the drive pulley, and actual transmission ratio. In such a system, the speed of changing of the transmission ratio up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshooting of the transmisson ratio occurs, which causes the driveability of the vehicle to reduce.
EP-A-No. 207603 discloses a system wherein desired transmission ratio is decided by a throttle valve opening degree and an engine speed to control the transmission ratio changing speed. Further, in an oil pressure control system, the line pressure is directly applied to a cylinder of the driven pulley and controlled line pressure is supplied to a cylinder of the drive pulley. In the system, the flow rate to the drive pulley cylinder is calculated, assuming that transmitting torque in the transmission is constant.
However, during the operation in accordance with the desired transmission ratio, the transmitting torque always varies. Thus, the line pressure is controlled in response to the variation of the transmitting torque. Since the line pressure is supplied to the drive pulley cylinder in order to control the transmission ratio, the variation of the line pressure affects necessarily the transmission ratio changing speed. As a result, actual transmission ratio does not coincide with desired transmission ratio.